Seed quality, as defined by the number of uniform usable plants obtainable from a seed batch, is becoming an ever more important trait in developed horticulture markets. Young plant raising is a highly technological activity in these markets and demands with respect to seed quality are therefore high. Reliable and consistently high seed quality is required for this. Also, germination under adverse conditions is an important seed quality trait. This means that one requirement for commercial success of seed varieties is consistent and robust seed quality. Currently however, seed quality of commercial varieties is not always stable and predictable.
Seed quality parameters are highly influenced by maternal environmental conditions during seed development. Given the volumes of seed needed and commercial feasibility, only limited controls of these conditions are possible. Therefore consistency of seed quality is limited by the susceptibility for maternal conditions. Research has shown that maternal environment can potentially affect all seed quality parameters, including uniformity and germination under adverse conditions. Decreasing the influence of maternal conditions would therefore lead to more consistent and robust seed quality, that provides advantages in both developed and developing markets.
Predictable and uniform seedling establishment is essential for the production of crops that are both sustainable and profitable. A key contributor to this predictability is the germination performance of seeds, which is influenced directly by seed dormancy and vigour. Dormancy per se (lack of germination in generally permissible conditions) is not considered to be a practical problem with many crop species, but low seed vigour (poor seed performance in practice) greatly influences not only the number of seedlings that emerge, but also the timing and uniformity of seedling emergence in all crops. The effects of this have a major impact upon many aspects of crop production that determine cost effectiveness and the inputs required, and there are also direct crop specific influences on marketable yield (Finch-Savage, 1995). Low seed vigour can result from seed deterioration and damage of many kinds and this has great commercial significance. However, there are also inherent differences in the initial vigour of the seed before it begins to deteriorate, but the genetic, molecular and physiological basis of this remains poorly understood.
Mutations in many genes have been identified that show phenotypes with altered seed germination performance and these have been instrumental in developing our current understanding of the control of germination (reviewed by Finch-Savage and Leubner-Metzger, 2006; Holdsworth et al., 2008a and 2008b). However, the relative impact of these genes in wild type or crop seeds is little understood and no clear candidates have been revealed that will form the basis of a discriminating test for seed vigour. An alternative source of genetic variation to laboratory induced mutations is available in natural populations and crop genotypes. Using this variation to identify QTL associated with seed vigour and then candidate genes influencing these traits may provide a route to identify practically important genes.
Both natural and crop plant variation has been exploited in quantitative genetic analyses of a range of seed vigour traits in tomato (Foolad et al., 1999), Brassica oleracea (Bettey et al., 2000, Finch-Savage et al., 2005) and Arabidopsis (Groot et al., 2000, Clerkx et al., 2004). Speed of seed germination QTLs have been identified in all three species.
The distinction between dormancy and low seed vigour in healthy non-aged seeds in terms of speed of germination, if one exists, is not understood and may have the same basis (Hilhorst and Toorop, 1997). In most situations, for example in Arabidopsis, physiological dormancy is not absolute, but seeds are conditionally dormant i.e. germination tends to be slow and is only possible in a limited range of environments. As dormancy is progressively lost, germination tends to speed up and becomes possible in a wider range of environments and can therefore appear like an increase in vigour.
Among the factors accounting for the establishment of seed germination and the regulation of seed dormancy, abscisic acid (ABA), a well-known plant hormone, plays an important role. ABA is in particular essential for the seed germination and seed maturation processes (for review, see Finkelstein et al. 2002) as it is responsible for the establishment of a period of seed dormancy. As for buds, it is important that the seeds do not germinate prematurely, for example, during unseasonably mild conditions prior to the onset of winter or a dry season. ABA in the seed enforces this dormancy. The dormancy is lifted only if the seed has been exposed to a prolonged cold spell and/or other appropriate environmental signal and if there is sufficient water to support germination. Besides its role in seed vigour, ABA also regulates many important aspects of plant life including the physiological responses to biotic threats and abiotic stresses like drought and dessication
There is thus a long-standing need for seed with a more reliable and constant seed vigour; especially with a timely-defined and uniform speed of germination, in order to provide seeds that germinate at a more constant rate, independently of the maternal conditions and whatever the external environmental conditions are. Such increased seed vigour would be of particular interest in cases where the seed is coated with a given preparation (chemical, biological), as it is usually observed a delay in seed germination. In consequence, seeds comprising sequences which enhance seed vigour, more particularly enhance the speed and the uniformity of the seed germination would be of primary importance to counteract the effect of the coating treatment, while still applying insecticides and fungicides.
Furthermore, while in many aspects, increasing the seed vigour would be a very useful and desired trait, it appears that, in some cases, decreasing the seed vigour would be of great interest. In particular in viviparous seed, decreasing seed vigour could be beneficial. Vivipary is defined as the germination of the seed while still on the mother plant or before drying and can occur both in immature and fully mature seed. Vivipary has been observed in many different crop species including Brassica crops (Ruan et al. 2000). Seeds comprising sequences which are able to decrease the seed vigour would thus be capable of delaying, if not removing, the non-desired vivipary phenotype.